This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We previously completed a large-scale analysis of physical protein interactions in yeast resulting in the identification of over 500 protein complexes (Nature 440, 637). More recently, we completed a screen for epistatically interacting genes in collaboration with Prof. Kevan Shokat, and have found that pairs of genes interacting positively correlate strongly with physical interactions, in particular between pairs of kinases/phosphatases and their substrate proteins. In this project, affinity tagged potential substrate proteins will be purified from yeast, digested into peptides, and enriched for phosphopeptides using the method of Trinidad et al., (Mol Cell Proteomics, in press). Enriched fractions will be analyzed by LCMS.